Effects of exogenous nitric oxide on neurotransmitter secretion and ionic currents in motor terminals

Abstract

Extracellular recording was used to study the effect of sodium nitroprusside, a donor of NO, on endplate transmitter release and ionic currents in frog cutaneous pectoris muscle. Exogenous NO inhibits induced transmitter secretion, and this effect is antagonized by extracellular Ca 2+. Exogenous NO increases potential-dependent outward potassium current and inhibits Ca2+-activated potassium current in the motor nerve terminals.Key Words: synapse; nitric oxide; nerve terminal ionic currents; transmitter secretion Nitric oxide (NO) is a key regulator of various physiological processes. Of particular importance is the role of NO in nerve cells and synapses in the central and peripheral nervous systems. NO acts as a second messenger of intracellular signaling, transmitter in cell-cell signal transmission, activity modulator in nerve cell neurotransmitter systems, etc. [5,6,8,12,14]. One of the methods to reveal the effects of NO and decipher the mechanism of its action is to increase the level of exogenous NO by using NO donors, i.e. the substances releasing NO in aqueous solutions [10]. Experiments with sodium nitroprusside (SN) as the NO donor showed that exogenous NO decreases the amplitudes of endplate potentials (EPP) evoked by motor nerve stimulation and frequency of miniature endplate potentials (MEPP) in frog neuromuscular synapse, without affecting their amplitude and dependence on membrane potential. These data suggest that exogenous NO has a presynaptic effect that moderates quantal secretion of the transmitter from motor terminals without modifying the function of chemosensitive ionic channels in the postsynaptic membrane [4,10]. However, the mechanisms of the presynaptic effects of NO are poorly understood.Our aim was to study the effects of exogenous NO on transmitter secretion and ionic currents in frog motor nerve terminals.Depa~rnent of Normal Physiology, Medical University, Kazan' MATERIALS AND METHODS Experiments were carried out on isolated neuromus. cular preparations of cutaneous pectoris muscle o:Rana ridibunda at room temperature. In most experiments we used a low-calcium physiological solutior (pH 7.2-7.4) containing (in mM): 115 NaCI, 2.5 KCI 2.4 NaHCO3, 0.2-0.4 CaCI2, and 2 MgC12. Some experiments were carried out with a standard solution (in mM): 115 NaCI, 2.5 KCI, 2.4 NaHCO3, and 1.8 CaCI 2. Blockade of action potentials and muscle contraction, as well as attenuation of endplate ionic currents were performed by adding of tubocurarine (0.02 mM) into the perfusion solution. The evoked responses of nerve terminals, EPP, and MEPP were recorded with extracellular glass microelectrodes (tip diameter 2-5 la) filled with 1 M NaCl [1][2][3][4]. Quantal composition of the endplate currents was calculated by falling (probability) analysis [9]: m--In n/no, where n is the number of stimuli and n o is the number of realizations.The motor nerve was stimulated by suprathreshold stimuli at a rate of 1 pulse per second.The synaptic signals were recorded with the help of an L-1230 digitizer incorporated in ...